Sculpting faces for Animation
As already mentioned in the comments you need a low poly mesh with a good topology for animation. If you have sculpted the head with dyntopo or the remesher then you can retopologize it to get the low-poly mesh.
Alternatively, you can use a low-poly base mesh and use sculpt tools to get the shape that you are after.
The mouth is created in a closed/relaxed position because the lips get stretched when it opens. The mouth consists of edge loops that form the lips. If you extrude these edge loops towards the inside of the head, you can easily create an oral cavity.
Teeth and tongue are separate meshes. You can make your own object from them.